RIFFELL, J.A.*; ZIMMER, R.K.; Univ. of California, Los Angeles; Univ. of California, Los Angeles: Sex and the single cell: The relationship between flow microenvironment, gamete traits and fertilization success.

Fertilization rate is an important factor mediating the production of offspring and, hence, population dynamics. For marine organisms that broadcast their sperm and eggs into the sea, surprisingly little is known about the mechanisms controlling sperm/egg interactions. This study considered hydrodynamic effects on fertilization in three invertebrate species. Although all three species cohabit rocky reefs in temperate kelp forests, each one occupies a different flow microenvironment. Experiments were performed in laminar-shear flows that simulated important aspects of small-scale turbulence within natural habitats. Fertilization rates were determined over a wide range of gamete concentrations and shears using a Taylor-Couette apparatus and a new application of infrared laser and computer-assisted video imaging technologies. For red abalone (Haliotis rufescens), shears of < 0.1/s maximized fertilization rates. In contrast, fertilization rates were maximized for red urchin (Strongylocentrotus franciscanus) at 1/s, and for purple urchin (S. purpuratus) at > 10/s. Based on field measurements, shears maximizing fertilization rates were those characterizing natural flow microenvironments of each species. Moreover, as a function of shear, fertilization rates of each species were well explained by theoretical gamete encounter models that took into account cell shapes, sizes, and sperm swimming speeds. Microenvironmental gradients in flow properties thus appear to constrain fertilization rates, exert strong selective pressures and, hence, drive evolution of gamete traits.